Loudspeaker enclosures



Nov. 26, 1963 A. H. ROBERTS 3,112,006

LoUDsPEAKER ENcLosuREs Filed Feb. s, 1961 m5 C: R;

SPEAKER lu i INVENTOR. ALFRED H.RoBERTs TQM# E FRELENCY P3 c, ATTORNEYUnited States Patent O 3,112,006 LOUDSPEAKER ENCLOSURES Alfred H.Roberts, 1615 Monk Road, Gladwyne, Pa. Filed Feb. 8, 1961, Ser. No.87,909 1 Claim. (Cl. 181-31) This invention relates to loudspeakerenclosures.

in my prior application for patent for loudspeaker enclosures, SerialNo. 803,856, tiled April 3, 1959, now U.S. Patent No. 2,978,060, thereis disclosed a loudspeaker enclosure which has certain properties of ahorn, of a Helmholtz resonator or basssreflex enclosure, and of a pipefolded once, but which does not satisfy all the criteria for any one ofthese lthree types of couplers.

In accordance with the present invention, a loudspeaker enclosure isprovided which has some lfeatures in common with the enclosure of myprior application, but which involves simplifications and impovements#and 1s particularly suitable for use as a stereophonic de-vice.

The essential differences with respect to the loudspeaker enclosure ofmy prior application are: (l) lthe capability of using a single widerange speaker with consequent reduction in cost, (2) the coaction of thesupporting panel with the wall and oor to form a corner-horn type o-fmouth improves the loading upon the speaker in the bass range, (3) thedownward disposition of the mouth which permits of transmitting theuttenmost low notes through coupling to .the floor, (4) the quarter waveair column is extended outwardly to the terminus of the mouth, reducingthe anti-resonant frequency with consequent extension of the bass range,(5) the area of the mouth terminus may be controlled by employing thecorrect elevation of the superstructure from the floor so that the areaof the mouth terminus can be made either equal or greater than the lareaof the intermediate passage with consequent increase in radiated soundpower, (6) the structure is compact and convenient for use in the home,and (7) the enclosure is particularly suited for use as -a stereophonicdevice.

It is the principal object of the present invention to provide aloudspeaker enclosure having an improved low audible frequency responsewhich economical of space and materials.

It is a further object of the present invention to provide a loudspeakerenclosure -which is efficient in the reproduction of sound yet iscapable of operationwith loudspeakers having diver-se operationalproperties.

It is a further object of the present invention to provide a loudspeakerenclosure in which the speaker fis supponted by a panel which functions`with the wall and floor of the room in which the encl-osure is used toprovide an improved type of discharge.

It is a further object of the present invention to provide a loudspeakerenclosure in which a preferred emf bodiment has provisions for thedirect delivery of the speaker output on one side of the speaker andindirect delivery from the other face of the diaphragm through anelongated path which includes 1an energy sink.

Other objects and advantageous features of the invention will beapparent from the description and claim.

The nature and characteristic features of the invention will be morereadily understood from the following description, taken in connectionwith the accompanying drawings forming pant thereof, in which:

FIGURE 1 Iis a vertical central sectional View of a preferred embodimentof the invention taken approximately on the line 1--1 of FIG. 2;

FIG. 2 is tan end elevational view of the loudspeaker enclosure of FIG.1 as seen from the lright hand end;

FIG. 3 is a front elevational view of another preferred embodiment ofIthe invention with the speaker facing in a forward direction; v

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FIG. 4 is a vertical sectional view taken approximately on the line 4-4of FIG. 3;

FIG. 5 is la diagrammatic view of the acoustical circuit as representedby the approximate electrical analogs; and

FIG. 6 is a diagrammatic view showing the variation With frequency ofIthe motional impedance of the loudspeaker housed in the loudspeakerenclosure of the present invention.

It should, of course, be understood that the description and drawingsherein `are illustrative merely, and that various modiiications andchanges can be made in the structure disclosed without departing fromthe spirit of the invention.

Like numerals refer to like parts throughout the several views.

Referring nou more particularly to FIGS. 1 and 2 of the drawings inwhich ya preferred embodiment of the invention is illust-rated, a pairof loudspeaker enclosures is shown as disposed at opposite ends or sidesof ra cabinet RPC in which a record player (not shown) or the like maybe disposed.

Each of the enclosures in accordance With the invention and in apractical and useful embodiment can have an exterior Width of the orderof 14 to 18 inches, an exterior depth of thefsame order, and a totalheight in feet of where fo is the primary resonance of the loudspeakerin free air in cycles per second.

The enclosure preferably includes a horizontal top wall panel 10, .andvertical side wall panel-s connected to and extending downwardly fromsaid top wall panel 10 and including vertical front `and rear ywallpanels 11 and 12, an inner vertical end wall panel A13, and an outervertical exterior or end Wall panel y15. The panel 12 may beadvantageously disposed against the vertical wall 14 of a room or otherenclosure and the panel 13 extends downwardly to the floor on which theenclosure is supported and with the other panel 13` of the other speakerenclosure can serve as a support. The panel 13 is prefenaibiy of a widthto serve a baille yand with the Wall 14 closes two vertical sides of thespace below the enclosure.

An interior baille is provided which includes a baiile panel 16extending inwardly and upwardly at an inclination with respect to thelower terminus of the panel 15, extends between the vertical wall panels11 and 12 and has an inclined panel 17 extending upwardly therefromwhich also extends to and between the panels 11 and 12. The panels 10,@11, 12, 15, '16 and 17 may be made of any desired material, such aswood, plywood, Veneered shipboard, ,and the like and panels of athickness of threequarters of fan inch have been found satisfactory.

'Ihe vertical supporting panel 13 may be made of the same material asthe panels just referred Ito, but is preferably of double thickness.

The interior of the enclosure preferably has reinforcing framing members'18 horizontally disposed therein. The panels, at their meeting margins,.are pretferalbly secured together in any desired manner so as to berigid and airtight therealong.

As shown in lFIGS. 1 and 2, the exterior panel 15 has a loudspeakerdiaphragm opening 19 therethrough and is adapted to support aloudspeaker '20' secured thereto in any desired manner with one surface20a of the speaker 'diaphragm facing outwardly and the other surface 20bbeing faced inwardly.

The loudspeaker 20 may be of -any preferred type such as anelectro-magnetic or an electro-static speaker.

f The loudspeaker 20 has a damping ring 21 of any 3 suitable materialapplied thereto :and held thereon. The damping ring 21 may be of anydesired construction but for the purposes of the present invention, adamping ring similar to that sho-wn in the application above identifiedis preferably e-mployed consisting of a ring 40 of glass wool snuglyfitted within a cowl 41.

The space within the wall panels 11, 1.2 and 15 and the supporting panel1.3 has an interior throat portion 36 bounded by the baflles 16 and 17,and the lower portions of :the fwaill panels 11, 12 and 15. The spacewithin the wall panels 11, 12, 13 and 15 also includes a space 31,functioning as an energy sink 31, communicating with the throat portion30 and bounded by the top panel 10 and upper portions of the panels 11,12, 13 and 15. The space within the wall panels 11, 12 and 15, and thesupporting panel 13 also includes an intermediate passage portion 35extending down-ward Ifrom the energy sink 31 and is bounded by the baies126 and 17 and the lower port-ions o-f the panels 11, 12 and 13, andcommunicates through a mouth portion 32 with the outer air, the mouthterminus 32a lying in the planes of the panels 11 and 15 projecteddownwardly to the iloor 14a of the room.

For -monaural systems Va speaker enclosure at only one end of thecabinet RPC will suffice, but for stereophonic use, two spacedenclosures are provided with a phonoradio compartment RPC therebetween.

Referring now to FIGS. 3 and 4, another orientation of the loud speaker2t) is shown with the speaker 20 lfacing away lfrom the wall 14 of theroom, with a horizontal baffle panel 16a and inclined baille panel 17aand with an opening or aperture 19b in the front wall panel 11a at whichone tace of the diaphragm of the loudspeaker 20 is exposed.

Reference will now be had to the -acoustical properties of the enclosurein accordance with the present invention.

The enclosure in accordance with the invention makes use of thequarter-wave action of a folded air column to establish ananti-resonance at and near Ithe primary free air resonance frequency ofa loudspeaker. In this respect, the enclosure resembles an organ pipe`closed at one end and folded once.

The proportions of the air column, most particularly at the fold, are,however, incompatible with those of an organ pipe or resonant tube andare in fact more those of a box. Above the top edge of the interiorbathe 17 and bounded by panels 10, 1-1, 12, 13 and 15 there exists anelastic volume of air which functions as an energy sink 31 at theantiresonance frequency. In this respect, the present inventionresembles the Helmholtz resonator, which -is also known in one of itsforms as the bassreex enclosure.

At |the inner end of the air column the interior throat portion 30 islocated, and at the outer end, beyond the mouth portion 32, lies themouth ter-minus 32a, peripheral to the solid angle defined by the roomwall `114, the iloor 14a and the lower part of the panel 13, and lyingin the planes of panels 11 and 15, as previously described. The mouthterminus 32a encompasses van area several times that of the interiorthroat portion 30, the latter area taken horizontally, and the ratio ofthese `areas multiplies, by impedance transformation, the useful loadupon the loudspeaker diaphragm surface 20h of FIG. 1. In this respect,the present invention resembles a small corner horn 'of the typefamiliar in this art.

'Considered as a pipe, the enclosure in accordance with the presentinvention is disqualified, however, by reason of the large reactiveimpedance at the fold (the energy sink 31) 'and by reason of the flaredorifice at the outer end of the air column.

Considered as a IHelmholtz resonator, the enclosure in accordance withthe present invention is disqualified because the antiresonance isestablished by the length of the air column, rather than by restrictionof area as in the vent of a bass-reflex enclosure. Within the energysink 31, .the air column and the compliant air volume are coterminousand in eifect fvibrate in two modes.

Considered as a horn, the enclosure in accordance with the presentinvention is disqualliiied (l) by the pronounced quarter-waveantiresonance of the air column and (2) by the shortness of the aircolumn with respect to the wavelengths transmitted. Horns must encompassat least several wavelengths from throat to mouth; and any air column,no matter what its ilare, becomes a simple discontinuity at its outerend when shorter in length than a quarter wave. (See L. L. Beranek,Acoustics, McGraw- Hill Book Co., 1954; page 2.72.)

-Undesired resonances, both longitudinal and transverse, must be avoidedand these may be eliminated or suppressed (l) by diverting higherfrequencies from the loudspeaker by means of an electrical crossovernetwork, (2) by proper design of the damping ring, which discriminatesagainst higher rfrequencies, and (3) by placement of acousticallyabsorbent material within the interior throat portion and/ or energysink portion of the coupler, either as a blanket lining or a stuiiing,and (4) by the inherent character of the invention, wherein thetransverse dimensions become comparable to a wavelength at frequencies-within the lower midrange of audibility, thus giving rise to dissonantweaker resonances in several modes and -in turn smoothing the frequencyresponse.

The present structure is a three-way acoustical hybrid whose behavior isregulated (l) by its structure and proportions and (2) by the use ofdissipative damping.

The electrical analogs shown in FIG. 5 aid in understanding theoperation of the present invention, making clear its `differences fromthe enclosures heretofore available.

Referring now more particularly to FIG. 5, the acoustical properties ofthe present invention are described in the electrical impedance analogas follows:

Cs=Compliance of the elastic suspension of the loudspeaker diaphragmRs=Resistance Iof the speaker suspension lumped with radiationres-istance encountered by the `face of the diaphragm 2Go Ms=Inertanceof loudspeaker moving system lumped 'with that of the air lyingimmediately in yfront of the Idiaphragm 20a Rd=Dissipative resistanceintroduced close to the diaphragm 20 by acoustically absorbent materialsuch as damping ring lZ1 Pc=Acoustical pressure acting upon thetransmission network U=Volume velocity produced by Pc Mt=lnertance ofinterior throat portion 30 Rb=Dissipative resistance within energy sink31 Cb=Complian`ce of air volume within energy sink 31 Mm=lnertance ofmouth portion 32 and mouth terminus 32a Rar-:Radiation resistance atmouth terminus 32a Ub=Volurne 'velocity through energy sink 31 Ua=Volumevelocity through mouth terminus 32a Ra and Rd vary approximately withthe second power of the frequency at the .frequencies of interesty(i.e., below 500 c.p.s.).

It is apparent that the network MtRbCbMm is a lowpass iilter of the Tconfiguration. It is known that such a network, when properlyproportioned, can be put to use as :an impedance transformer; when thenetwork is matched to the internal impedance of the driver at one endand to the load impedance at the other the maximum trans-fer of energyis achieved.

In the present invention, it is desired to extend the low frequencyresponse below the point at which it starts to fall olf in a closed box.This lower cutoff point occurs when the inertances of the loudspeakerand the associated air load resonate with the compliances of the speakersuspension and the closed box, in series. This resonance occurs at 'afrequency higher than the primary free air resonance of the loudspeaker.

Referring now more particularly to FIG. 6 wherein impedances expressedin decibels are plotted as ordinates and frequency in cycles per secondas abscissas, curve A illustrates the rise in the motional impedance ofa typical electromagnetic loudspeaker at its free air resonance pointf2. Curve B shows the similar ytree ai-r performance with a damping ring(such as 21 in FIGS. 1 and 2) attached to the speaker near the back Zlibof the diaphragm. Curve `C shows the variation 'in inotional impedanceof the same loudspeaker within the enclosure of FlGS. 1 and 2 with thedamping ring 2l in place. These inotional impedances appear as voltagesat the terminals of the loudspeaker woice coil. Four frequency pointsare manned in FflG. 6:

f1=series resonance of Ms, CS, Mt and Mm (of FlG. 5) f2=series resonanceof Ms with Cs in free air 3=series resonance of MS, Cs, Mt, and Cbf4=antiresonance of series branch Ms, Cs and Mt and shunt branch CDM,

he effect of the `damping ring 21, as shown in ourve B is clear. it hasreduced the impedance liu-nip by approximately two to one. Theantiresonant effect of the quarter wave air column as shown in curve Cis also clear. Since the eloeities of the diaphragm and the antiresonantair column at the throat are in phase opposition, the excursion of thediaphragm f2 is reduced to its normal level. This in turn reducesharmonie distortion arising `from nonlinearitiesin the loudspeakersystem and maximizes radiation at the mouth terminus 32a.

l claim: A loudspeaker enclosure for use along a vertical room wall andin spaced relation to the room oor having a plurality of spaced verticalWall panels including t-wo antenne side wall panels and front `and rearwall panels, said vertical wall panels being vertically connected andone of said side wall panels extending downwardlyto the room door, a topwall panel closing said vertical panels, a baffle nieniher connected torone of said vertical panels and to an opposed pair of said verticalpanels and extending upwardly and terminated spaced relationship to saidtop wall panel, said baile providing on one `face thereof with itsconnected vertical panels an interior throat portion .and on the otherface thereof with said opposite pair of connected vertical panels andwith an opposing vertical panel an intermediate passage portion, thespace within said vertical panels and above said baille and below saidtop wall panel providing an energy sink. in communication with saidinterior throat portion `and with sain intermediate passage portion,said energy sink being larger than said interior throat portion, thelspace below said vertical wall panels and to the roonn door providing amouth por-tion terminating immediately below said vertical panels, and aloufdspearcer mounted at one side of said throat portion and having atleast one diaphragm face thereof in `corranunication with said throatportion, said interior throat, energy sink, intermediate passage andmouth portions providing a folded air column no greater in length than aquarter wave of sound in air at the primary resonance frequency of said`loudspeaker.

References Cited in the file of this patent UNTED STATES PATENTS2,765,864 Glenn Oct. 9, i956 2,866,513 White Dec. 30, 1958 2,985,229Ferlsins May 30, 1961 OTHER REFERENCES Saunat (German printedapplication) L2G-852 VH1 aZlaZ, Aug. 30, 1956.

